There is a commercial trend to develop alternative energy resources that are more durable and cheaper than current mainstream technologies. This movement is important in light of the increasing demand for energy and the gradual depletion of global fossil fuels. Fuel cells have been long considered a promising solution for the production of clean and efficient energy.
The electrochemical conversion that takes place in fuel cells is not limited by the Camot efficiency which restricts the efficiencies of internal combustion engines (ICE). A fuel cell is two to three times more efficient at converting fuel to power than ICEs. Compared to batteries, fuel cells offer the advantage of continuous power as long as the reactant fuel and oxidant are supplied. This eliminates the time consuming recharging procedure.
Furthermore, fuel cells are environmentally friendly. As fuel, such as hydrogen, is fed into the anode compartment, a metal catalyst oxidizes the hydrogen gas into protons and electrons. The only emission when hydrogen is used as the fuel is water. In fuel cells, the electrode assemblies of both the anode and the cathode contain a metal catalyst (e.g., platinum) supported by a conductive material. Some PEM fuel cells use a diffusion layer on both electrodes to help distribute gases evenly across the electrode surfaces. Fuel cells use an electrolyte between the cathode and the anode. Fuel cells that employ proton conducting electrolyte membranes are referred to as “proton exchange membrane fuel cells” (PEMFC). Reactions take place where the electrolyte, gas, and an electrode are in contact with one another (multiphase boundary). The protons are transferred through the electrolyte material to the cathode, while the electrons are conducted through an external circuit (from the anode) to the cathode via an electrically conductive material. At the cathode an oxidant, such as oxygen, diffuses through the electrode where it reacts with the electrons and protons to form water. The operation of PEM fuel cells produces electricity, water, and heat.
In addition to using hydrogen, or materials that reform to hydrogen, PEM fuel cells can also operate directly with fuels such as methanol. In this case, methanol is introduced directly into the anode compartment and is internally reformed. These types of fuel cells are called direct methanol fuel cells (DMFC).
Among the various types of fuel cell technologies, PEMFC and DMFC are the preferred power sources for residential, portable, and transport applications because of their relatively lower operation temperature and quick start up times. Both PEMFCs and DMFCs use a proton conducting membrane as the electrolyte and composite electrode assemblies consisting of platinum based electrocatalysts and carbon. The main difference between PEMFCs and DMFCs is the type of fuel each of them use and their emissions. While PEMFCs use gaseous hydrogen as fuel and emit only water, DMFCs use methanol as fuel and emit water and carbon dioxide.
One of the most significant components of PEMFCs and DMFCs is the proton exchange membrane (PEM). As previously noted, the proton exchange membrane acts as an ionic conductor between anode and cathode and separates the fuel and oxidant. Several polymer electrolyte membranes are being explored as proton exchange membranes in PEM fuel cells. Presently, both PEMFC and DMFC use expensive hydrated perfluorosulfonic acid based membranes as the electrolyte because of their excellent chemical, mechanical and thermal stability and relatively high proton conductivity of around 0.08 Scm−1 in the hydrated state.
The leading commercial perfluorosulfonic acid based membrane is Nafion® (Nafion® is a trademark of E. I. DU PONT DE NEMOURS and Company Corporation, Delaware). Nafion®, which is described, for example, in U.S. Pat. No. 4,330,654 is fabricated by melting tetrafluoroethylene and perfluorovinyl ethersulfonyl fluoride together, shaping the mixture, and then hydrolyzing the melt to yield the ionic sulfonate form.
While perfluoronated ionomer membranes, such as Nafion® membranes, are effective in PEM fuel cells they have limitations. Among these limitations are reduced proton conductivity at elevated temperatures (>80° C.), high osmotic expansion, limited maximum operating temperature (<100° C.), high methanol permeability, and high cost.
Nafion® has inherent water management difficulties when operating above 80° C. resulting in decreased proton conductivity. These membranes need to be adequately humidified to provide satisfactory proton conductivity. This is due to the hydrophilic nature of the sulfonic acid groups attached to the polymer backbone and the necessity to hydrate the ionic clusters. However, when the membrane temperature exceeds the boiling point of water, the membrane dehydrates and experiences a dramatic decrease in the proton conductivity. Consequently, perfluoronated ionomer membranes are not regarded as suitable for fuel cell applications above 100° C. Conversely, operation of PEMFCs at elevated temperatures (T>100° C.) can provide several significant advantages. For example, the higher operating temperature can provide faster reaction kinetics, better efficiencies, reduce or eliminate Pt-based catalyst poisoning by carbon monoxide impurity in the fuel, and possibly allow the use of less expensive non-platinum alloy or transition metal oxide catalysts.
Several strategies have been employed to increase the operating temperature of Nafion® and Nafion® like membranes. Many have tried sol-gel or other processes to infiltrate the porous structure of Nafion® with components that will increase its performance at elevated temperatures. Staiti et al and Tazi et al impregnated Nafion® with phosphotungstic acid and silicotungstic acid/thiophene, respectively, which increased proton conductivity and hydration levels at temperatures up to 120° C. (See, P. Staiti, “Proton Conductive Membranes Based on Silicotungstic Acid/Silica and Polybenzimidazole”, Materials Letters, 47 (2001) 241-246, and B. Tazi et al, “Parameters of PEM Fuel Cells Based on New Membranes Fabricated From Nafion®, Silicotungstic Acid and Thiophene”, Electrochimica Acta, 45 (2000) 4329-4339). Others including P. Costamagna et al and Park et al demonstrated that Nafion® doped with zirconium hydrogen phosphate provided similar results. (See, P. Costamagna et al, “Nafion® 115/Zirconium Phosphate Composite Membranes for Operation of PEMFCs Above 100° C., Electrochimica Acta, 47, 2002, 1023-1033 and Y. Park et al, “Proton Exchange Nanocomposite Membranes Based on 3-Glycidoxypropyltrimethoxysilane, Silicotungstic Acid and Zirconium Phosphate Hydrate”, Solid State Ionics, 145, 2001, 149-160). However, by using Nafion® as the base material, these membranes are still very expensive. In addition, some of these additives leach out of the membrane structure during fuel cell operations, which limits their utility.
When used in DMFCs, high methanol permeability is another significant deficiency exhibited by perfluoronated ionomer membranes (e.g., Nafion® like membranes). Methanol crossover is much more prevalent than hydrogen crossover, especially at concentrations above 10 wt %. This is primarily a result of the liquid concentration gradient. To minimize crossover, some researchers have incorporated additives into Nafion® or vaporized methanol before introducing it to the anode compartment. This solution, however, does not address Nafion's® expensive cost or inherent disposition to methanol crossover.
One of the other major drawbacks of perflouronated membranes such as Nafion® is its high cost. Due to its relatively complicated and time-consuming manufacturing process, Nafion® is expensive ($700 per square meter at the time of this writing). Typically, Nafion® membranes represent 10-15% of the total cost of a single PEM fuel cell or stack of fuel cells. It is generally accepted that if Nafion® we re to continue to represent the leading membrane candidate for PEM fuel cells, its cost must come down substantially before these cells can become competitive in the fuel cell market.
A variety of alternative membranes have been considered for solving the technical limitations of Nafion® in PEM fuel cells, but none of these alternatives has demonstrated sufficient advantages to replace Nafion® as the membrane of choice. One alternative membrane incorporates Nafion® or a Nafion®-like polymer into a porous polytetrafluoroethylene (Teflon®) structure. These membranes are available under the trade name Gore-Select® from W. L. Gore & Associates, Inc. and they are described in U.S. Pat. Nos. 5,635,041, 5,547,551 and 5,599,614. Other alternative membranes are available under the trade names Aciplex® from Asahi Chemical Co. and Flemion® from Asahi Glass. Due to their polyfluoronated structures, these alternative membranes exhibit many of the same deficiencies as Nafion®, namely, limited ionic conductivity at elevated temperatures, dehydration or drying up, and fuel crossover.
Composite ion exchange membranes with a low expansion, durable polymer impregnated with a high proton conductive polymer described, for example, in U.S. Pat. No. 6,248,469 represent another alternative. The main disadvantage of such membranes is that they loose some of the cross sectional area of the proton conductive material due to the presence of the inactive support.
Another alternative to Nafion® membrane employs polybenzimidazole polymers (PBI) that are infiltrated with phosphoric acid. These have been used as ion exchange membranes in PEM fuel cells and are described in U.S. Pat. Nos. 5,716,727 and 6,099,988. These membranes permit PEM fuel cells to operate at higher temperatures above 130° C., and exhibit lower osmotic expansion than Nafion®. However, the concentrated acid leaches out from the PBI pores as water is produced during the electrochemical fuel cell process, thereby dramatically reducing the proton conductivity and electrochemical performance with time. The leached phosphoric acid may also react with other components in the fuel cell stack.
Finally, more recent research has led to unique formulations and designs of ion exchange membranes. For example, Chen et al showed that incorporation of montmorillonite and lithium triflate into poly(ethylene oxide) (PEO) enhanced the ionic conductivity of the electrolyte by nearly 16 times compared to unmodified PEO. See Chen et al, “The Novel Polymer Electrolyte Nanocomposite Composed of poly(ethylene oxide), lithium triflate and mineral clay”, Polymer, 42 (2001) 9763-9769. However, the increased proton conductivity values in these fuel cells were still substantially lower than those produced by fuel cells using Nafion®. Similarly, Aranda et al created a membrane by combining poly(ethylene oxide) and ammonium exchanged montmorillonite, but the membrane also exhibited low ion conductivity.
Based on the foregoing, there is a demonstrated need to develop inexpensive and higher performing alternatives to existing proton exchange membranes. Additionally, there is a need for a more cost effective membrane fabrication method.